Treatment and prevention of Hepatitis C virus (HCV) infections remains a major challenge for controlling this worldwide health problem; existing therapies are only partially effective and no vaccine is currently available. Hepatitis C (HCV) virus infects more than 170 million people worldwide and is the leading cause of liver transplants. Existing treatments, including ribavirin and pegylated interferon alpha, are effective only in approximately 50 percent of patients and have substantial side effects. The development of more effective HCV treatments is hampered by the lack of a good small animal model, the inability to stably culture the virus in tissue culture cells, and the high viral mutation rate [1-3]. The availability of an HCV replicon system has allowed the study of HCV replication, host-cell interactions and evaluation of anti-viral agents, and more recently, a transgenic chimeric humanized mouse liver model was developed that allows full HCV infection [4-7]. Moreover, the use of in vivo imaging of HCV IRES-dependent reporter systems has facilitated efficient evaluation of delivery and inhibition by anti-HCV agents in mouse liver over multiple time points using the same animals [8].
RNA interference is an evolutionarily conserved pathway that leads to down-regulation of gene expression. The discovery that synthetic short interfering RNAs (siRNAs) of about 19-29 base pairs can effectively inhibit gene expression in mammalian cells and animals without activating an immune response has led to a flurry of activity to develop these inhibitors as therapeutics [9]. Chemical stabilization of siRNAs results in increased serum half life [10], suggesting that intravenous administration may achieve positive therapeutic outcomes if delivery issues can be overcome. Furthermore, small hairpin RNAs (shRNA) have also shown robust inhibition of target genes in mammalian cells and can be easily expressed from bacteriophage (e.g. T7, T3 or SP6) or mammalian (pol III such as U6 or H1 or polII) promoters, making them excellent candidates for viral delivery [11].
Efforts have been made to find effective nucleic acid-based inhibitors against HCV, as existing treatments are not fully effective (reviewed in [4, 12]). These efforts include traditional antisense oligonucleotides, phosphorodiamidate morpholino oligomers [8], ribozymes, and more recently siRNAs. It has been shown that siRNAs can effectively target HCV in human tissue culture cells [13-19] and in animal systems [20].